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(#) Implied locale in date format

!!! WARNING: Implied locale in date format
   This is a warning.

Id
:   `SimpleDateFormat`
Summary
:   Implied locale in date format
Severity
:   Warning
Category
:   Correctness
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   1.1.0 (February 2015)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/reference/java/text/SimpleDateFormat.html
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/DateFormatDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/DateFormatDetectorTest.kt)

Almost all callers should use `getDateInstance()`,
`getDateTimeInstance()`, or `getTimeInstance()` to get a ready-made
instance of SimpleDateFormat suitable for the user's locale. The main
reason you'd create an instance this class directly is because you need
to format/parse a specific machine-readable format, in which case you
almost certainly want to explicitly ask for US to ensure that you get
ASCII digits (rather than, say, Arabic digits).

Therefore, you should either use the form of the SimpleDateFormat
constructor where you pass in an explicit locale, such as Locale.US, or
use one of the get instance methods, or suppress this error if really
know what you are doing.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/LocaleTest.java:32:Warning: To get local formatting use
getDateInstance(), getDateTimeInstance(), or getTimeInstance(), or use
new SimpleDateFormat(String template, Locale locale) with for example
Locale.US for ASCII dates. [SimpleDateFormat]
    new SimpleDateFormat(); // WRONG
    ----------------------
src/test/pkg/LocaleTest.java:33:Warning: To get local formatting use
getDateInstance(), getDateTimeInstance(), or getTimeInstance(), or use
new SimpleDateFormat(String template, Locale locale) with for example
Locale.US for ASCII dates. [SimpleDateFormat]
    new SimpleDateFormat("yyyy-MM-dd"); // WRONG
    ----------------------------------
src/test/pkg/LocaleTest.java:34:Warning: To get local formatting use
getDateInstance(), getDateTimeInstance(), or getTimeInstance(), or use
new SimpleDateFormat(String template, Locale locale) with for example
Locale.US for ASCII dates. [SimpleDateFormat]
    new SimpleDateFormat("yyyy-MM-dd", DateFormatSymbols.getInstance()); // WRONG
    -------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/LocaleTest.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import java.text.*;
import java.util.*;

public class LocaleTest {
    public void testStrings() {
        System.out.println("OK".toUpperCase(Locale.getDefault()));
        System.out.println("OK".toUpperCase(Locale.US));
        System.out.println("OK".toUpperCase(Locale.CHINA));
        System.out.println("WRONG".toUpperCase());

        System.out.println("OK".toLowerCase(Locale.getDefault()));
        System.out.println("OK".toLowerCase(Locale.US));
        System.out.println("OK".toLowerCase(Locale.CHINA));
        System.out.println("WRONG".toLowerCase());

        String.format(Locale.getDefault(), "OK: %f", 1.0f);
        String.format("OK: %x %A %c %b %B %h %n %%", 1, 2, 'c', true, false, 5);
        String.format("WRONG: %f", 1.0f); // Implies locale
        String.format("WRONG: %1$f", 1.0f);
        String.format("WRONG: %e", 1.0f);
        String.format("WRONG: %d", 1.0f);
        String.format("WRONG: %g", 1.0f);
        String.format("WRONG: %g", 1.0f);
        String.format("WRONG: %1$tm %1$te,%1$tY",
                new GregorianCalendar(2012, GregorianCalendar.AUGUST, 27));
    }

    @android.annotation.SuppressLint("NewApi") // DateFormatSymbols requires API 9
    public void testSimpleDateFormat() {
        new SimpleDateFormat(); // WRONG
        new SimpleDateFormat("yyyy-MM-dd"); // WRONG
        new SimpleDateFormat("yyyy-MM-dd", DateFormatSymbols.getInstance()); // WRONG
        new SimpleDateFormat("yyyy-MM-dd", Locale.US); // OK
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/DateFormatDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `DateFormatDetector.testSpecifyingLocale`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("SimpleDateFormat")
  fun method() {
     ofPattern(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("SimpleDateFormat")
  void method() {
     ofPattern(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection SimpleDateFormat
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="SimpleDateFormat" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'SimpleDateFormat'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore SimpleDateFormat ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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